Can brushing improve the color of teeth, or just maintain?

Yes.

You could use a whitening toothpaste, they do help, but it will not be as noticeable as if the dentist had whitened them.

Your foolish if you don't see a dentist. If you have plaque built up under the gum line you will end up losing your teeth. The yellowing may be helped with whitening strips but DO NOT overdo it. Follow the directions carefully. You need dental cleanings and inspection at the very least once a year. I go twice a year. It's worth it to keep your teeth.